<!DOCTYPE html>
<html lang="zh">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style type="text/css">
        #div01{
            color:red;
        }
        .div02{
            color:blue;
        }
        div{
            color:yellow !important;
        }
    </style>
</head>
<body>
<div id="div01" class="div02">
    我是div里的内容！
</div>

</body>
</html>